CN114899689A - Linear cavity mode-locked polarization-maintaining fiber laser with high environmental stability - Google Patents

Abstract

The application relates to the field of industrial automation, and provides a high-environmental-stability line cavity mode-locking polarization-maintaining optical fiber laser, which consists of a pumping source and a laser oscillation cavity; the marked reflector can be replaced by a semiconductor saturable absorption mirror, a laser oscillation cavity with the reflector as an end mirror and a pumping source form a NPE mode-locked fiber laser with a linear cavity, and the semiconductor saturable absorption mirror serves as the laser oscillation cavity with the end mirror and the pumping source to form a hybrid mode-locked fiber laser with the linear cavity. According to the polarization maintaining optical fiber, the Faraday optical rotator and the total reflection mirror are added at one end of the linear cavity, perfect self-adaptive compensation for group velocity mismatch and linear phase bias is realized by exchanging fast and slow axis signal light which is transmitted back and forth in the polarization maintaining optical fiber, and the problem that the NPE technology in the polarization maintaining optical fiber is difficult to evolve is solved. And a real saturable absorber is added to assist mode locking so as to reduce the mode locking threshold.

Description

Linear cavity mode-locked polarization-maintaining fiber laser with high environmental stability

Technical Field

The application relates to the field of industrial automation, in particular to a high-environment-stability line cavity mode-locking polarization-maintaining fiber laser.

Background

The mode locking technology is one of the most common methods for realizing ultrashort laser pulse output in the prior art, and mainly comprises active mode locking and passive mode locking. The active mode locking is realized by adding an artificial modulator in a cavity, and is usually limited by the response time of the modulator, the output pulse width can reach the magnitude of nanosecond (ns) and picosecond (ps), and the ultra-short pulse of femtosecond (fs) magnitude is difficult to obtain; the passive mode locking uses a saturable absorber effect, the mode locking is realized by utilizing the self structure of the resonant cavity rather than external modulation, the response speed is high, and the output pulse width of the output pulse can easily realize the output of pulses of several picoseconds or even femtosecond magnitude. In addition, passive mode locking has the advantages of relatively simple structure, low cost, good stability and the like, and is more widely applied to various mode locking oscillators.

The passive mode locking mechanism usually adopts a saturable absorber to realize mode locking. Existing mature passive mode locking mechanisms include material-based true saturable absorber mode locking and optical kerr effect-based artificial saturable absorber mode locking. The real saturable absorber mode locking technology shows reliable self-starting mode locking capability, but has the problems of long recovery time, complex and difficult preparation, low damage threshold, performance degradation after long-time operation and the like, and needs to be replaced periodically to maintain the performance of the laser, so that the problem of difficult later maintenance and the like exists when the mode locking fiber laser is constructed by only using the real saturable absorber. In contrast, the artificial saturable absorber has the advantages of extremely fast recovery time (several femtoseconds), extremely high damage threshold, flexible and convenient structural design, and the like, and therefore becomes a preferred scheme of the mode-locked fiber laser in the aspects of generating high power, high energy and ultrashort pulse output.

Two common forms of the current artificial saturable absorber mainly exist, namely a nonlinear polarization rotation evolution (NPE) mode locking technology and a nonlinear loop mirror (NALM) mode locking technology. NPE technology has been extensively studied in the field of ultrafast fiber lasers over the past decades, but its most successful implementation is in non-polarization maintaining fiber, which makes it highly susceptible to external environmental changes (such as temperature changes, acoustic vibrations, mechanical vibrations, etc.), thereby causing unstable mode locking and affecting the long-term stability of the laser, which also becomes a stumbling block for commercializing NPE mode-locked fiber lasers. In contrast, the NALM mode locking technology is independent of polarization evolution, and even polarization evolution cannot occur, so that the NALM mode locking technology is naturally applicable to polarization-maintaining fiber lasers, but a mature 8-shaped oscillation cavity based on the NALM technology needs to accumulate certain nonlinear phase shift to realize mode locking, so that the cavity length cannot be too short, pulse output with high repetition frequency cannot be realized, and output energy is also limited.

In summary, the NPE technique shows a greater advantage in terms of output performance. However, the long-term stability of the optical fiber laser is not as good as that of the NALM technology, and the NPE technology needs to be applied to the polarization-maintaining optical fiber, the polarization-maintaining optical fiber is not sensitive to the external environment, the modulation instability caused by the weak birefringence effect of the non-polarization-maintaining optical fiber can be avoided, the overall environmental stability of the laser cavity is further improved, and the optical fiber laser is an indispensable element for realizing the commercialization of the optical fiber laser. To successfully realize NPE in the polarization-maintaining optical fiber, a key scientific problem to be overcome is how to solve the problem of polarization evolution failure caused by time delay introduced by different group speeds of a fast axis and a slow axis, which becomes a research hotspot in recent years. In an article published by an optical rapid report (Optics Letters) in an Szczepanek topic group in 2017, the article is named as 'ultra fast laser mode-locked using nonlinear polarization main locking fibers', polarization maintaining fibers are combined with an NPE mode locking technology, a multi-section polarization maintaining fiber is used for angle fusion to form an artificial saturable absorber, and a full polarization maintaining annular cavity optical fiber laser based on NPE mode locking is built, so that pulse output with the pulse width of 150fs is obtained, the repetition frequency of 20.54MHz and the output energy of 0.85 nJ. In a ring cavity polarization-maintaining fiber laser built by a general Generation of a strained pulses from an annular polarization-maintaining and main induced Er-doped mode-locked fiber laser utilization of a general university of east China in 2019 Applied Physics Express, three sections of polarization-maintaining erbium-doped fibers are adopted for angle fusion to realize NPE mode locking, and tensile pulses with the repetition frequency of 90.5MHz and the pulse width of 90fs are obtained. In the NPE mode-locked ring-cavity polarization-maintaining optical fiber laser which is set up in the Applied Physics Express, namely the NPE mode-locked ring-cavity polarization-maintaining optical fiber laser, the related topic of the Chinese science and technology university in 2021 sets up in the Applied Physics Express, two Polarization Beam Splitters (PBS) are arranged between two sections of polarization-maintaining gain optical fibers to replace the angle fusion between the optical fibers, and the dissipative soliton pulse with the repetition frequency of 2.36MHz and the pulse energy of 21.2nJ is obtained, but the structure is very complicated.

The device is the polarization maintaining fiber laser of annular chamber structure more than, two first studies in order to compensate the pulse and walk away from, the problem of group speed mismatch, need carry out the angle butt fusion with the multistage optic fibre to the multistage optic fibre, and every section optic fibre length needs accurate control in proportion, this is a difficult problem at the technical level, thereby the spectrum anomalous phenomenon of output has appeared, still can face the mode locking when patting the optic fibre near splice point slightly and lose the lock problem, can't realize the long-term stable output of pulse. The latter solution requires the special customization of two kinds of PBS devices with optical fiber structures, which makes the structure complicated and the cost high. The technical problems of the scheme are that large-scale mass production cannot be realized, and the commercial development of the optical fiber laser is not facilitated.

Disclosure of Invention

In order to solve the technical problems existing in the prior art, the main object of the application is to provide a high-environmental-stability line cavity mode-locking polarization-maintaining optical fiber laser, which adopts a spliced structure, is flexible and diverse in product forms, can flexibly change different splicing modes according to customer requirements, and is combined into different products.

In a first aspect, the application provides a high-environmental-stability mode-locked polarization-maintaining fiber laser with a linear cavity, which comprises a pumping source and a laser oscillation cavity, wherein the pumping source irradiates pumping light to a second reflecting mirror through a dichroic mirror in sequence, and the second reflecting mirror is used for turning back signal light and reflecting the signal light to a first Faraday optical rotator, a first quarter wave plate and a polarization beam splitter through the dichroic mirror and then reaches the first reflecting mirror.

As a further scheme of the invention, a linear cavity polarization-maintaining fiber laser based on NPE mode locking is formed by a laser oscillation cavity taking a first reflector as an end mirror and a pumping source; a semiconductor saturable absorption mirror and a lens combination are used for replacing a laser oscillation cavity and a pumping source of the first reflecting mirror part, so that a hybrid mode-locked linear cavity polarization-maintaining fiber laser is formed.

As a further aspect of the present invention, the polarization maintaining fiber is a combination of two ends of a polarization maintaining gain fiber respectively welded with a segment of passive fiber.

As a further scheme of the invention, the optical system also comprises a spatial optical path, wherein the spatial optical path comprises a Faraday rotator and a reflecting mirror and is used for carrying out adaptive compensation on the optical path.

In a further aspect of the present invention, in the spatial light path, the pump light is pumped into the gain fiber through a dichroic mirror.

As a further scheme of the invention, the device also comprises a phase shifter for assisting the mode locking self-starting.

As a further aspect of the present invention, the combination of the phase shifters may be a non-reciprocal phase-biasing arrangement of the first faraday rotator, the first one-half wave plate, and the first one-quarter wave plate.

As a further aspect of the present invention, the polarization beam splitter is located at a position where two polarization states interfere with each other, the polarization beam splitter provides an output port for the laser, and the polarization beam splitter is located in the laser oscillation cavity, wherein a reflection end of the polarization beam splitter is used as the output port to monitor the horizontally polarized light passing through the polarization beam splitter after being reflected back from the optical fiber.

As a further aspect of the present invention, the auxiliary mode locking part is a combination of a semiconductor saturable absorber mirror and an aspheric lens, and a second quarter-wave plate is inserted between the polarization beam splitter and the lens, wherein the aspheric lens is a third collimating lens.

Compared with the prior art, the invention has the following beneficial effects:

the application provides a high-environmental-stability mode-locked polarization-maintaining fiber laser with a linear cavity, wherein a Faraday optical rotator and a holophote are added at one end of the linear cavity, the perfect compensation of group velocity delay and linear phase offset is realized by exchanging optical paths in a round-trip process, and the problem of realizing an NPE (neutral point optical) technology in a polarization-maintaining optical fiber is solved by using a self-adaptive compensation method; and a real saturable absorber is directly added to assist mode locking so as to reduce the mode locking threshold.

The high-environmental-stability linear cavity polarization-maintaining fiber laser provided by the invention utilizes a combination scheme of the Faraday optical rotator and the reflector to compensate pulse time delay, so that the difficulties of angle welding and length control are avoided, the linear structure cavity is relatively simple, the coupling difficulty of space light is low, too large maintenance cost is not needed in the later period, and the commercial development is favorably realized. In addition, the invention also introduces devices such as a semiconductor saturable absorption mirror, a non-reciprocal phase biaser and the like to assist mode locking, thereby further improving the self-starting capability of mode locking.

These and other aspects of the present application will be more readily apparent from the following description of the embodiments. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present application. In the drawings:

FIG. 1 is a schematic structural diagram of an NPE mode-locked fiber cavity polarization-preserving fiber laser with high environmental stability in an embodiment of the present application;

fig. 2 is a schematic diagram of an alternative structure of a semiconductor saturable absorber mirror as an end mirror in a hybrid mode-locked fiber cavity polarization-preserving fiber laser with high environmental stability in an embodiment of the present application.

FIG. 3 is a schematic structural diagram of an embodiment of a polarization maintaining optical fiber of the line cavity mode-locked polarization maintaining fiber laser with high environmental stability in FIG. 1;

FIG. 4 is a spectrogram of an NPE mode-locked fiber cavity polarization-preserving fiber laser with high environmental stability in stable mode locking in the embodiment of the present application.

Fig. 5 is a spectrogram of a hybrid mode-locked fiber cavity polarization-preserving fiber laser with high environmental stability when stable mode locking is realized in the embodiment of the present application.

Fig. 6 is a graph of an autocorrelation trace of an output pulse in a hybrid mode-locked fiber cavity polarization-preserving fiber laser with high environmental stability in an embodiment of the present application.

Fig. 7 is a power monitoring diagram of stability measurement of the laser in a hybrid mode-locked fiber cavity polarization-preserving fiber laser with high environmental stability in 4 hours in the embodiment of the present application.

Fig. 8 is a schematic diagram of stability measurement of a hybrid mode-locked linear cavity polarization-maintaining fiber laser with high environmental stability in 4 hours under eight sets of spectral conditions measured at intervals of 30 minutes in an embodiment of the present application.

Reference numbers in the figures:

the optical fiber polarization maintaining device comprises a 1-pumping source, a 2-dichroic mirror, a 3-first Faraday optical rotator, a 4-first one-half wave plate, a 5-first one-quarter wave plate, a 6-polarization beam splitter, a 7-first reflector, an 8-first collimating lens, a 9-first optical fiber connector, a 10-polarization maintaining optical fiber, a 101-polarization maintaining gain optical fiber, a 103-passive optical fiber, a 11-second optical fiber connector, a 12-second collimating lens, a 13-second Faraday optical rotator, a 14-second reflector, a 15-phase shifter, a 16-second one-quarter wave plate, a 17-third collimating lens, a 18-semiconductor saturable absorber, a 19-real saturable absorber mode locking and a 20-laser oscillation cavity.

The objectives, features, and advantages of the present application will be further described with reference to the accompanying drawings.

Detailed Description

Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

In the description of the present invention, the meaning of a plurality of means is one or more, the meaning of a plurality of means is two or more, and larger, smaller, larger, etc. are understood as excluding the number, and larger, smaller, inner, etc. are understood as including the number.

In the description of the present invention, the consecutive reference numbers of the method steps are for convenience of examination and understanding, and the implementation order between the steps is adjusted without affecting the technical effect achieved by the technical solution of the present invention by combining the whole technical solution of the present invention and the logical relationship between the steps.

In the description of the present invention, unless otherwise explicitly defined, terms such as set, etc. should be broadly construed, and those skilled in the art can reasonably determine the specific meanings of the above terms in the present invention in combination with the detailed contents of the technical solutions.

From the appearance of the first solid laser to the development of fiber lasers nowadays, the laser pulse width realizes the breakthrough from nanosecond level to femtosecond level, and the output performance of the laser is also developed towards higher and better direction. Compared with a solid laser, the ultrafast fiber laser has the advantages of good beam quality, high thermal management efficiency, flexible and compact structure, low maintenance cost and the like, so that the ultrafast fiber laser has important application in the fields of military affairs, scientific research, industry, medical treatment and the like. The ultra-short pulses of different wave bands can be output by changing the doping ion types of the gain optical fiber in the laser oscillation cavity, and the working wave band of the optical fiber laser is covered from the visible light wave band to the middle and far infrared wave bands, so that different requirements of different fields are met. For example, ytterbium-doped fiber lasers can generate laser light in the 1.0 μm band and have important applications in industrial fields (e.g., cutting, welding, etc.); the erbium-doped fiber laser can generate laser with a wave band of 1.5 mu m, the wave band is a low-loss communication window of an optical communication wave band, the transmission loss in the optical fiber is only 0.2dB/km generally, and the research on a stable light source with the wave band of 1.5 mu m plays an important role in optical fiber communication; meanwhile, light with the wave band of 1.5 mu m is positioned in a human eye safe wave band, and the wave band is suitable for a plurality of targets (such as vehicles, ships, cement buildings and the like)) The contrast with the background is large, and the method is very attractive in military field application such as laser radar, target identification and the like. As another example, a thulium or holmium doped fiber laser may generate ultrashort laser pulses in the 2 μm band encompassing CO in the "fingerprint" region of the molecule ₂ ，H ₂ O and NO ₂ Equal molecular absorption spectrum can form a high-sensitivity gas sensor for realizing remote atmospheric sensing, and because more than 75% of human tissues are composed of water, 2-micron-band ultrashort laser pulses are also widely applied to the aspects of medical treatment (such as laser scalpels, tissue excision and the like); meanwhile, ultrashort pulses with the wave band of 2 microns are used as a seed source of the laser, and the laser wavelength can be easily expanded to the middle and far infrared wave bands with the wavelength of more than 10 microns by utilizing the optical nonlinear conversion effect (such as difference frequency, supercontinuum generation and other technologies). With the development of the optical fiber preparation technology and related fields, the optical fiber laser with better and better performance is continuously excavated, and the polarization maintaining optical fiber laser with high stability and easy mode locking improves the overall environmental stability of the laser in application, makes an important contribution to the development of the optical fiber laser, and promotes the development of the optical fiber laser.

The traditional fiber laser based on NPE mode locking generally uses non-polarization-maintaining fiber as a fiber component, and because NPE is based on the mode locking mechanism of the interference principle, the mode locking state is easily influenced by the external environment and cannot stably run for a long time. The polarization maintaining optical fiber insensitive to environmental influence is used to replace a non-polarization maintaining optical fiber, two polarization states transmitted in the optical fiber are not influenced mutually, the nonlinear phase change is only related to power and is not influenced by the environment, and the overall environmental stability of the laser can be greatly improved. Therefore, the fiber laser based on the polarization maintaining fiber has natural high stability and is beneficial to realizing commercial application. With the continuous development of optical fiber preparation technology, polarization-maintaining rare earth element doped optical fibers are gradually commercialized, and polarization evolution cannot be realized due to the problem that time delay occurs to optical pulses of fast and slow axes caused by different group speeds of the fast and slow axes of the polarization-maintaining optical fibers, so that an NPE mode locking technology is difficult to apply to the polarization-maintaining optical fibers, and related reports of a full-polarization-maintaining NPE mode locking femtosecond optical fiber laser are few. The ring cavity structure reported at present utilizes the angle fusion between the optical fibers to solve the problem of pulse time delay, but the technology needs to accurately control the fusion angle and reduce the preparation efficiency on one hand, and needs to accurately control the length of the optical fibers on the other hand, which is also a challenging task. Secondly, the problem that the mode locking threshold is high usually exists in the realization of the full polarization-preserving mode-locking femtosecond fiber laser by only utilizing the NPE technology, the starting in a multi-pulse state is usually realized by needing larger pumping power, and then the stable and reliable single-pulse operation can be realized by further reducing the pumping power. In addition, the high mode locking threshold limits the application of the fiber laser, improves the requirement on a pumping source and increases the use cost. Therefore, solving the problem of high mode-locking threshold is also a crucial scientific problem.

Therefore, the application provides a high-environmental-stability mode-locked polarization-maintaining fiber laser with a linear cavity, wherein a Faraday optical rotator and a total reflector are added at one end of the linear cavity, the perfect compensation of group velocity delay and linear phase offset is realized by exchanging optical paths in a round-trip process, the technical problem is solved by a self-adaptive compensation method, and a real saturable absorber is directly added to assist mode locking so as to obviously reduce the mode locking threshold.

The high-environmental-stability polarization maintaining fiber laser with the linear cavity provided by the invention utilizes the combination of a Faraday rotator and a reflector to compensate the pulse time delay, so that the difficulties of angle welding and length control are avoided, the linear structure cavity is relatively simple, the coupling difficulty of space light is low, too large maintenance cost is not needed in the later period, and the commercial development is favorably realized. In addition, the invention also introduces devices such as a semiconductor saturable absorption mirror, a non-reciprocal phase biaser and the like to assist mode locking, thereby further improving the self-starting capability of mode locking.

Referring to fig. 1, an embodiment of the present invention provides a high-environmental-stability mode-locked polarization-maintaining fiber laser with a linear cavity, which is composed of a pumping source 1 and a laser oscillation cavity 20, wherein the pumping source 1 irradiates pumping light through a dichroic mirror 2 in sequence to a second reflecting mirror 14 through a first collimating lens 8, a first fiber connector 9, a polarization-maintaining fiber 10, a second fiber connector 11, a second collimating lens 12, and a second faraday optical rotator 13, and the second reflecting mirror 14 is used for turning back the pumping light and reflecting the pumping light to the first faraday optical rotator 3, the first quarter-wave plate 4, the first quarter-wave plate 5, and a polarization beam splitter 6 through the dichroic mirror 2 and then to the first reflecting mirror 7.

In some embodiments of the present application, the laser oscillation cavity 20 with the first mirror 7 as an end mirror and the pump source 1 constitute a line cavity polarization-maintaining fiber laser based on NPE mode locking.

In the embodiment of the present application, the first reflecting mirror 7 is an end mirror oscillator with a total cavity length of 3.95m, wherein the optical fiber 10 has a partial length of 3.55m, and the spatial optical path has a partial length of 0.4 m.

In order to effectively reduce the radius of a light spot of a spatial light path and ensure that the light spot is not limited by the clear aperture of a spatial optical device, the polarization-maintaining optical fiber 10 is a combination of a polarization-maintaining gain optical fiber 101 and a passive optical fiber 103, the polarization-maintaining gain optical fiber 101 is a 2.9m polarization-maintaining holmium-doped optical fiber (PM-HDF, IXF-HDF-PM-8-125), the numerical aperture is 0.16, two ends of the polarization-maintaining gain optical fiber are respectively welded with a section of passive optical fiber 103 with low bending loss for a 2 mu m waveband, the model is Thorlabs SM2000, and the numerical aperture of the passive optical fiber is 0.12.

The space optical path comprises a Faraday rotator and a reflector, and is used for rotating the light of the fast axis and the slow axis of the polarization maintaining optical fiber by 90 degrees and then returning the light to the cavity of the optical fiber.

In the embodiment of the application, the combination of the Faraday optical rotator and the reflector is adopted to solve the difficulty of angle fusion and accurate control of the length of the optical fiber in an all-fiber structure, and the linear phase delay can be completely counteracted on the other hand, so that the nonlinear phase shift is continuously accumulated, the hole burning effect in the cavity is eliminated, the mode locking self-starting threshold value is reduced, and the problem of group velocity mismatch is solved.

In some embodiments of the present application, in another part of the spatial light path, pump light is pumped into the polarization maintaining fiber via dichroic mirror 2(DM, Layertec splitter 103086), and the pump light is provided by a homemade Thulium Doped Fiber Laser (TDFL) which can provide 1948nm continuous light of up to 1.34W.

In some embodiments of the present application, the first faraday rotator 3, the first quarter-wave plate 4 (HWP) and the first quarter-wave plate 5 (QWP) constitute a non-reciprocal phase bias (non-reciprocal phase bias) device, i.e. a phase shifter 15, for assisting mode-locking self-start.

In some embodiments of the present application, the polarization beam splitter 6 is located at a position where two polarization states interfere, and the polarization beam splitter 6 provides an output port for the laser and is located in the laser oscillation cavity 20, wherein the reflection end of the polarization beam splitter 6 is used as the output port to monitor the horizontally polarized light that passes through the polarization beam splitter 6 after being reflected back from the optical fiber.

Referring to fig. 3, when the pump power was increased to 1.07W, multi-pulse mode-locking was achieved, and then when the pump power was decreased to 303mW, a stable single-pulse soliton mode-locking state was achieved, where the port output average power was 0.1mW, and the mode-locked pulse spectrum is shown in fig. 3 with a center wavelength of 2128nm and a 3-dB full width at half maximum of about 6.5 nm. The monitored spectrum is slightly asymmetric due to the small power. The output spectrum has obvious Kelly sideband, which is a typical characteristic of soliton mode locking, and the cavity net dispersion is calculated to be-0.827 ps by calculating the relation between the Kelly sideband and the central wavelength ² 。

In some embodiments of the present application, referring to fig. 2, in the mode-locked polarization-maintaining linear fiber laser with high environmental stability, the first mirror 7 may be replaced by a combination of a saturable absorber mirror 18 and a third collimating lens 17, and the laser oscillation cavity 20 and the pump source 1 of the first mirror 7 portion may be replaced by a combination of the saturable absorber mirror 18 and the third collimating lens 17 to form a hybrid mode-locked cavity polarization-maintaining fiber laser.

In an embodiment of the present application, the true saturable absorber mode-lock 19 is partially composed of a combination of a semiconductor saturable absorber mirror 18(SESAM, batospam-2150-8-10 ps-4.0-25.0s-c) and an aspheric lens 16, which is a third collimating lens 17, and is inserted between the polarization beam splitter 6 and the lens.

After replacing the first reflector 7 with the semiconductor saturable absorber mirror 18 and the third collimating lens 17, the pumping threshold required by the mode locking self-starting of the laser is greatly reduced, in addition, the second quarter-wave plate 16 can be added, and the output power of the control port of the wave plate can be rotated in a large range, so that the mode locking self-starting capability is improved.

When the pumping power reaches 420mW, the mode locking of the laser is started automatically to obtain multi-pulse mode locking, and then the pumping power is reduced to 365mW, so that stable single-pulse soliton mode locking output can be realized. Compared with the method without the SESAM, the pump power required by mode locking self-starting is reduced from 1.07W to 420mW (reduced by about 60%), and the mode locking threshold is obviously reduced. The mode-locked output spectrogram of the mode-locked laser is shown in fig. 4, and the output average power of the port at this time is 4.86 mW. By comparing fig. 4 with fig. 3, the visible spectrum shape does not change much, which means that even if a true saturable absorber is added, the nonlinear polarization evolution mode locking mechanism still plays a dominant role in the polarization maintaining mode locking fiber oscillator.

The process of replacing the first mirror 7 with the true saturable absorber mode-locking 19 results in a slight change in the optical path, and the center wavelength of the spectrum shifts to 2110 nm. The addition of the semiconductor saturable absorption mirror 18 not only reduces the pumping power of the mode locking self-starting threshold value by 60%, but also greatly improves the self-starting capability of the mode locking, and the advantage enables the application to extract higher-quality mode locking pulses from the port.

Meanwhile, the semiconductor saturable absorber mirror 18 also has a filtering effect on stray light in the cavity, because most of the stray light has low power and is absorbed when passing through the real saturable absorber, the mode locking spectrum becomes smoother, and the full width at half maximum of 3-dB is relatively larger and is about 7 nm.

Fig. 5 is an autocorrelation trace of a soliton pulse. The port output pulse autocorrelation graph can be perfectly fitted by using a hyperbolic secant curve, so that the soliton pulse output by the laser is proved, and the actual pulse width is 1.13ps after estimation.

In addition, in order to characterize the laser stability, the present application also measured the long-term power stability of the laser. The output power at the output port was monitored over time using a laboratory electrical probe power meter (Thorlabs PM1000& S148C). Fig. 6 shows the power stability of the laser for 4 hours of continuous operation, even though the laser was turned on five hours before measuring the power stability, the monitored laser stability was good for 4 hours, with a rms jitter of the output power of 0.14%, which is less than that of a non-polarization maintaining fiber laser oscillator. In addition, the present application also records the change of the pulse spectrum within 4 hours at intervals of 30 minutes, and as shown in fig. 7, the total of 8 sets of spectra have extremely high coincidence degree, which indicates the stability of the oscillator. In addition, the spectral shapes of fig. 8 and 5 are different because we intentionally adjust the angle of the second quarter-wave plate 16 in experiments, and change the output state to further prove that the laser designed by us has flexible output and can maintain excellent stability under different output states.

The polarization-maintaining holmium-doped optical fiber in the embodiment of the application can be replaced by other polarization-maintaining high-concentration rare earth element-doped gain optical fibers, so that ultrashort laser pulse output in different working wave bands is realized. For example, a 2 μm waveband laser output is realized using a polarization maintaining thulium doped fiber (PM-TDF) or a polarization maintaining thulium holmium co-doped fiber (PM-THDF); using a polarization maintaining ytterbium-doped fiber (PM-YDF) to realize laser output of a 1 mu m waveband; the laser output of 1.5 μm waveband was realized using polarization maintaining erbium doped fiber (PM-EDF) and polarization maintaining erbium ytterbium co-doped fiber (EYDF).

The true saturable absorber in the present application can be replaced with other two-dimensional materials such as graphene, carbon nanotubes, phospholene, topological insulators, etc. The function of the semiconductor saturable absorption mirror is the same as that of the semiconductor saturable absorption mirror 18, and the semiconductor saturable absorption mirror has the advantages of easiness in self-starting, simple structure, stable performance, low mode locking threshold value, short response time and the like. A hybrid mode locking mechanism is formed in the laser, which is beneficial to reducing the mode locking self-starting threshold value, improving the self-starting capability and playing a role in smoothing the spectrum.

The phase shifter composed of the half wave plate, the quarter wave plate and the Faraday optical rotator in the application is a non-reciprocal phase shifter for realizing mode-locking self-starting of the laser, and can be replaced by any other commercial devices or optical devices for realizing mode-locking self-starting and non-reciprocal phase shifting.

The optical fiber part in the application adopts a full negative dispersion optical fiber, can be converted into soliton pulse under the combined action of intracavity negative Group Velocity Dispersion (GVD) and nonlinear effect, and is limited by that the single pulse energy of the soliton area theorem is in the order of 0.1 nJ. The whole optical fiber part can be replaced by positive dispersion optical fiber or the combination of negative dispersion and positive dispersion optical fiber, and a high-environment-stability linear cavity mode-locking polarization-maintaining optical fiber laser with higher output mode-locking pulse energy is constructed.

The above description is only a preferred embodiment of the present application, and not intended to limit the scope of the present application, and all modifications of equivalent structures and equivalent processes, which are made by the contents of the specification and the drawings of the present application, or which are directly or indirectly applied to other related technical fields, are included in the scope of the present application.

Claims (10)

1. A high-environmental stability line cavity mode-locked polarization-maintaining fiber laser is characterized by comprising a pumping source (1) and a laser oscillation cavity (20);

the pump light is guided into the laser oscillation cavity (20) by the pump source (1) through the dichroic mirror (2), and reaches the second reflecting mirror (14) through the first collimating lens (8), the first optical fiber connector (9), the polarization maintaining optical fiber (10), the second optical fiber connector (11), the second collimating lens (12) and the second Faraday optical rotator (13) in sequence; the second reflecting mirror (14) is used for turning back the signal light, reflecting the signal light to the first Faraday optical rotator (3), the first one-half wave plate (4), the first one-quarter wave plate (5) and the polarization beam splitter (6) through the dichroic mirror (2) and then reaching the first reflecting mirror (7).

2. The high-environmental-stability mode-locked polarization-maintaining fiber laser with the linear cavity according to claim 1, wherein the NPE mode-locked polarization-maintaining fiber laser is formed by the laser oscillation cavity (20) with the first reflector (7) and the second reflector (14) as end mirrors and the pump source (1).

3. The high-environmental-stability mode-locked polarization-maintaining fiber laser with the linear cavity according to claim 2, wherein the polarization-maintaining fiber (10) is a combination of a passive fiber (103) welded to two ends of a polarization-maintaining gain fiber (101).

4. The high environmental stability line cavity mode-locked polarization-maintaining fiber laser of claim 2, further comprising a spatial optical path, wherein the spatial optical path comprises a faraday rotator and a mirror for adaptive compensation of the optical path.

5. The high environmental stability mode-locked polarization-maintaining fiber laser with linear cavity according to claim 4, wherein in the spatial optical path, the pump light is pumped into the gain fiber through a dichroic mirror (2).

6. The high environmental stability line-cavity mode-locked polarization-maintaining fiber laser of claim 1, further comprising a phase shifter (15) for assisting mode-locking self-start.

7. The high-environmental-stability line-cavity mode-locked polarization-maintaining fiber laser according to claim 6, wherein the combination of the phase shifters (15) is a non-reciprocal phase-bias device composed of a first Faraday rotator (3), a first one-half wave plate (4) and a first one-quarter wave plate (5).

8. The high environmental stability line cavity mode-locked polarization-maintaining fiber laser according to claim 1, wherein the polarization beam splitter (6) provides an output port for the laser, and the polarization beam splitter (6) is located in the laser oscillation cavity (20), wherein the reflective end of the polarization beam splitter (6) is used as the output port for monitoring the horizontally polarized light transmitted through the polarization beam splitter (6) after being reflected back from the optical fiber.

9. The high-environmental-stability mode-locked polarization-maintaining fiber laser with the linear cavity according to claim 1, wherein the laser oscillation cavity (20) and the pump source (1) which replace the first reflector (7) by a semiconductor saturable absorber mirror (18) as an end mirror constitute a hybrid mode-locked polarization-maintaining fiber laser with the linear cavity.

10. The high-environmental-stability line-cavity polarization-maintaining fiber laser according to claim 9, wherein in the hybrid mode-locked line-cavity polarization-maintaining fiber laser, a semiconductor saturable absorber mirror (18) is placed in a laser oscillation cavity (20) in combination with an aspheric lens, and a second quarter-wave plate (16) is inserted between the polarization beam splitter (6) and the lens, wherein the aspheric lens is a third collimating lens (17).

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